Pusan Perimeter

The Pusan Perimeter (4 August-18 September 1950) was a large-scale battle of the Korean War that saw 140,000 United Nations troops make a final stand at Pusan in southwestern South Korea against the 98,000-strong North Korean army, which had overrun the South Korean capital of Seoul and much of the country. For six weeks, the UN forces managed to hold the line against repeated North Korean assaults, and on 15 September a counterattack with a landing behind enemy lines at Inchon forced the poorly-supplied and battered North Koreans to withdraw. The battle was the high water mark of the North Korean advance, and the war would later devolve into stalemate.

Background
On 25 June 1950, North Korea declared war on South Korea and launched an all-out invasion of the country. The United States had ground forces stationed in South Korea, and due to a mutual defense treaty, America was obligated to take the side of South Korea against the communist and Soviet-backed North Korea in the "Korean War". The ensuing conflict would see South Korea and United Nations coalition forces, primarily from the United States and United Kingdom, fight a defensive war against the North Koreans in an attempt to maintain South Korea's independence. The surprise that the Korean People's Army had achieved left the UN coalition forces as sitting ducks, and the UN suffered several defeats at the hands of the North Koreans in early July. The North Koreans were able to enter the South Korean capital of Seoul, and they won the Battle of Taejon against the Americans. On 24 July 1950, the Americans began a campaign to hold their perimeter to the south of Seoul, with the South Korean port of Pusan being the impromptu capital of South Korea and the main base of the UN forces in the country. US reservists were called up and thousands of US troops dispatched to South Korea to hold the perimeter.

Kumi and Taejon campaign
The campaign began with a North Korean attack on the town of Kumi, where they were met with stiff resistance from American armored forces. The battle there coincided with South Korea's attempt to hold the North Koreans at Taejon, almost completely without US support, as most US troops were embatted at Kumi or were disembarking at Pusan at the time. On 26 July, Kumi was captured by the Americans, but the South Koreans' attempt to defend Taejon faltered as more North Korean troops were deployed to the battle. The Americans were unable to commit any ground forces to help the South Koreans, as they had to push north along the west coast; without US support, the South Koreans lost Taejon on 29 July and Chonju shortly afterwards. The North Koreans had whipped the South Korean forces along the coast, demoralizing the UN forces, and some believed that the North Koreans would be able to crush the Americans as well.

The Yongju-Chongju Campaign and the Chonju counterattack
All seemed lost when the Americans embarked on their campaign to clear the Yongju-Chongju Gap. A large concentration of North Korean forces between the towns of Yongju to the west and Chongju to the east threatened to reverse the Americans' gains, so a large American armored assault was ordered against the North Korean forces in the gap on 26 July. On 27 July, the Americans secured Yongju and secured Wonju and Chech'on on 30 July, pushing the North Koreans back. More North Korean troops were deployed to fight the Americans there, resulting in the South Koreans being able to mount a counterattack alongside the Americans at Taejon and Chonju. Chonju was recaptured on 4 August, allowing for a full counterattack to be mounted at Taejon. In Operation Marshall, the US and South Korean forces crushed the last North Korean forces along the western coast of South Korea and pushed up to Seoul on 11 August. The campaign resulted in a great UN victory, with the south being liberated. The aftermath of the campaign would see UN forces advance further north and take Pyongyang and reach the Yalu River by December.